Low Hormone Therapy Adherence Challenges Breast Cancer Radiation Omission Strategies

Fewer than 30 percent of early-stage breast cancer patients consistently maintain their prescribed five-year endocrine therapy after undergoing a lumpectomy, according to a large U.S. insurance claims analysis presented at the American Society for Radiation Oncology (ASTRO) Annual Meeting. The findings show that patients who skip radiation therapy face a significantly higher risk of experiencing substantial lapses in their long-term medication routine compared to those who received radiation therapy.

Insurance Claims Analysis Reveals Five-Year Adherence Rates

Researchers analyzed Merative MarketScan commercial and multistate Medicaid claims data from 17,025 patients aged 23 to 64. Every patient in the study group had hormone receptor-positive, early-stage breast cancer, underwent a lumpectomy, and filled at least one endocrine therapy prescription. While 89 percent of the study cohort also received radiation therapy, the longitudinal pharmacy data revealed a steady drop in medication adherence over time.

Investigators calculated adherence using a medication possession ratio to determine whether patients had enough medication available to follow their prescribed treatment. The standard threshold required patients to have medication available for at least 80 percent of the measured period. Overall, approximately 28 percent of patients remained continuously adherent at that benchmark through the five-year follow-up. Although annual adherence appeared higher—with 59 percent to 75 percent of patients meeting the standard in individual years—the multiyear tracking showed a cumulative decline.

Radiation Omission and Younger Age Linked to Medication Lapses

The study identified specific factors associated with larger treatment gaps. Patients who did not receive radiation therapy were twice as likely to fall below a 50 percent medication-possession threshold over time and showed a significantly greater likelihood of dropping below 60 percent and 70 percent benchmarks. Younger age also correlated with lower adherence rates.

Radiation therapy after a lumpectomy is a standard approach to lower the risk of cancer returning in the treated breast. For selected older patients with lower-risk disease, clinical trials have established that radiation can be omitted safely without compromising overall survival, though local recurrence risks increase. These omission strategies depend heavily on patients continuing endocrine therapy, which blocks or lowers the hormones fueling hormone receptor-positive breast cancer, for the full five years.

Side effects such as hot flashes, fatigue, joint pain, bone health impacts, and changes in memory and concentration often make multiyear medication regimens difficult to maintain.

Our findings are a reminder that doctors prescribing endocrine therapy for five years and patients actually taking the medication for five years are not the same thing.

Shannon Jiang, MD, lead study author and radiation oncology resident physician, Washington University School of Medicine in St. Louis

Clinical Implications for Long-Term Breast Cancer Care

The study authors noted that the analysis could not definitively determine why omitting radiation therapy correlated with lower endocrine therapy adherence. Variations in health care access, treatment preferences, or the frequency of contact with oncology care teams during radiation treatments may contribute to the observed differences.

When radiation therapy is omitted, endocrine therapy becomes an especially important part of the treatment strategy for preventing recurrence, according to study leadership. Investigators emphasize that clinicians need to have realistic conversations with patients considering radiation omission about what long-term medication routines and daily quality of life will involve.